The present invention relates to a method of repairing a labyrinth seal of a diffuser in a primary pump of a nuclear power station.
A nuclear power station primary pump comprises a drive shaft for rotating a wheel, and above said wheel and concentrically about said shaft, a heat exchanger forming a thermal barrier, a thermal barrier cover comprising a peripheral skirt provided at its base with a rim extending beneath the heat exchanger, and a pump diffuser comprising a peripheral skirt concentric with said peripheral skirt of the thermal barrier cover and provided at its base with a circular end wall extending beneath the rim of said body.
This type of primary pump also has a ring known as the xe2x80x9clabyrinthxe2x80x9d ring disposed between the heat exchanger and the shaft, and a body having the heat exchanger and the peripheral skirt of the thermal barrier cover fixed thereto. The body is connected to the thermal barrier cover by link elements.
In addition, the circular end wall of the diffuser has a central bore through which the shaft passes, where it is provided with a succession of circumferential and superposed rings forming a lip seal of the labyrinth type.
The labyrinth seal is thus provided in a solid part, and the circular end wall of the diffuser carrying said labyrinth seal is placed in a transition zone between the hot water of the primary circuit which is at a temperature of about 300xc2x0 C. and the cooled zone of the pump which is at a temperature of about 40xc2x0 C.
The circular end wall of the diffuser, and consequently also the labyrinth seal, are thus both subject to a large temperature gradient which encourages the formation of thermal fatigue cracks on the lips of the seal.
An object of the invention is thus to provide a method of repairing the labyrinth seal of a primary pump diffuser in a nuclear power station, said diffuser comprising a peripheral skirt provided at its base with a circular end wall having an orifice provided with said labyrinth seal, and the method consisting in:
making a bore in the circular end wall of diameter greater than the orifice so as to withdraw a central portion of the end wall and eliminate the labyrinth seal;
reducing the thickness of the remaining portion of the end wall in the vicinity of the bore;
forming a chamfer on the edge of the bore;
positioning a replacement part in said bore, the replacement part having a central orifice, and the peripheral edge of said part being provided with a chamfer;
providing a continuous and leakproof bead of welding between the two chamfers;
making flush the flash on the bead of welding; and
making lips to form a new labyrinth seal in the central orifice of the replacement part.
According to other characteristics of the invention:
the axis of symmetry of the chamfers of the remaining portion of the circular end wall and of the replacement part is parallel to the axis of the central orifice;
the axis of symmetry of the chamfers of the remaining portion of the circular end wall and of the replacement part forms an angle relative to the axis of the central orifice that is acute and that has a vertex situated beneath said circular end wall;
the bottom face of the replacement part is situated substantially in the same plane as the bottom face of the circular end wall of the diffuser; and
the bottom face of the replacement part is situated above the bottom face of the circular end wall of the diffuser.